DARPA is the Defense Advanced Research Projects Agency, a US government organization that performs research in military technology. One of their current projects, the Fine Detail Optical Surveillance (FDOS), hopes to develop optical equipment sensitive enough to literally "a needle moving along the surface of a haystack." That phrase is actually in the specs:

The program can be described as developing the technology and systems analogous to that required for the rapid imaging and identification, without the need for scanning or focusing of the optical receiver, of a needle moving along the surface of a haystack, where the location and type of needle on the haystack is uncertain.

A needle would have a fairly limite range of possible lengths and shapes. That would seem a much simpler problem than looking for the more generalised field of "threat" which I presume the final article is intended to detect.